There are many instances wherein it is desirable to use a holder for navigation charts, maps, data sheets and the like, and to illuminate such materials so that they can be read during night flight operations. During such operations, it is usually desirable to eliminate glare and stray light conditions; consequently, the illumination of such charts may create a problem. Part of the difficulty resides in the inverse square law of illumination from a point source. Without proper corrective measures, a light bulb positioned away from the material to be illuminated and out of the observer's line of sight tends to illuminate those surfaces nearest the bulb to a much greater extent than more remote areas are illuminated. This problem has been addressed, for example, in U.S. Pat. No. 3,246,133, issued to Hensleigh. That patent discloses the use of light distributing plates having complementary conical cavities, with a bulb effectively on the axis of the cone.
A further problem is that it is difficult to provide uniform illumination of a sufficient level at the reading surface, especially of the ANVIS green light required during night operations. It is especially difficult to get appropriate illumination when night vision goggles are being used, where low contrast conditions within the cockpit are essential.